1. Field of the Invention
The present invention relates generally to misfire detection in internal combustion engines, and more specifically to a system and method that adjusts for systematic irregularities in measured engine velocities caused by torsional flexing of the crankshaft during rotation.
2. Related Art
One way to detect misfires in internal combustion engines is to measure crankshaft speed and observe fluctuations in speed. Detection of deviations in crankshaft velocities (as manifested by abnormal acceleration values) from expected normal crankshaft velocities is an indication of misfire. Deviations in acceleration are determined over nominally equal successive intervals of crankshaft rotation, referred to as "Profile-Ignition-Pick-Up intervals" (PIP intervals). A PIP signal is a digital signal received from a sensor, which detects specified positions of rotation of a crankshaft mounted wheel during engine rotation. PIP intervals are also known as combustion intervals, which are equal in length (but not necessarily phase) to the angular rotation between top dead centers of adjacent cylinders in the firing order.
Ideally, during normal operation, an engine will produce a series of PIP transition signals whose periods indicate the average crankshaft velocity during a substantial portion of the power stroke for each of the cylinders in the engine. The crankshaft velocity will either remain constant (zero acceleration), increase (positive acceleration) or decrease (negative acceleration) depending on whether the engine is operating at steady state, accelerating or decelerating, respectively. For example, if a normal engine is operating under steady-state operation (no acceleration), then it is expected to produce an acceleration value of near zero over successive PIP intervals. However, if a particular cylinder in an engine produces a sufficiently negative value during steady-state operation, then this occurrence will be interpreted as a misfire condition, since a zero value is expected as an output for all cylinders in a normal engine during steady state operation.
Accordingly, misfire detectors in general look for individual cylinders yielding acceleration values different from the local norm of all cylinders, where the local norm depends upon the operating condition (i.e., steady state, acceleration, or deceleration, etc.). The problem is that individual cylinders in normal engines tend to yield values of velocity, which differ slightly from the local norm of all cylinders in a systematic manner according to cylinder number. In a normally operating engine, this will interfere with the misfire detection system's ability to detect abnormal behavior due to misfire.
There are at least two sources of such cylinder-specific-irregularities. The first is discussed in Dosdall et al. U.S. Pat. No. 5,117,681, incorporated herein by reference. Dosdall et al. deals with systematic irregularity arising from PIP spacing of the wheel. If the wheel which serves as the position encoder on the crankshaft is even slightly irregular in PIP-interval spacing (e.g., a few tenths of a degree difference), then a normal engine operating at constant true PIP-to-PIP velocity (steady state) will appear to be experiencing subtle velocity changes (hence, non-zero acceleration values). The velocity changes will appear to coincide with the particular cylinders associated with the irregular PIP intervals. Although the degree of impact that a given wheel error has on the acceleration calculation is strongly rpm-dependent, the error itself is fixed, and so it can be empirically determined at any operating condition, even deceleration.
Dosdall et al. prefer to employ defueled coast down when sensing encoder errors. The reason is to avoid uneven acceleration during data collection, since with no fuel, all cylinders are unpowered. Application of the process specified in Dosdall et al. to coast down data yields a set of values which indicates the actual PIP-interval spacings of the wheel relative to the nominal values (i.e., assumed equal spacing). There are only n/2 unique correction values derived in this manner, since crankshaft mounted wheels typically have half as many PIP-intervals as engine cylinders (n). Each individual correction value is used twice per engine cycle.
The second problem, subject of the present invention, is that even under normal operation conditions the crankshaft will produce different amounts of speed-up and slow-down because of the non-rigid (torsional) behavior of the crankshaft. Since the crankshaft is not rigid, it produces subtle oscillations (due to crankshaft flexing) in the PIP signal (systematic "noise"). This noise tends to camouflage true misfires and can cause an erroneous indication of one or more cylinders as having misfired even when engine operation is in fact normal. For example, cylinders at the front of a crankshaft might affect the speed of the crankshaft at the measuring point slightly differently than cylinders at the rear. The effects of crankshaft-torsional-flexing can occur when the power is cut-off (as in the Dosdall et al. patent), because inertial torques produce uneven motions (acceleration). In general, the effects of such torsional variations on the calculated acceleration values are most pronounced at higher engine speeds as manifested in typical engine data.
There is no prior technique able to adequately solve the foregoing problems consistent with PIP interval data collections used in misfire detection schemes. Therefore, what is needed is a mechanism that compensates for torsional effects in crankshaft speed calculations to enable accurate misfire detection.